Considerable research is ongoing to identify packaging materials and encapsulation techniques for protecting integrated circuitry in an endless variety of applications ranging from outer space to implantations in the human body. Each unique application exposes the integrated circuitry to a different environment, which in most cases includes elements that are potentially harmful to integrated circuit chips. Many applications expose the integrated circuitry to trace chemicals which, in time, can permeate the chips to prejudice performance and operating life. Even the diffusion of ions into the chip semiconductor body has deleterious consequences. For example, when applied to implanted prostheses, integrated circuitry is exposed to extracellular fluid which approximates a 0.15 normal NaCl solution at 37.degree. C. and is quite hostile to semiconductor chip structures.
In my U.S. Pat. No. 4,198,444, issued Apr. 15, 1980 and assigned to the instant assignee, the disclosure of which is specifically incorporated herein by reference, there is provided an encapsulation process for surface passivating and hermetically sealing large thyristors and other high voltage power semiconductor devices. As disclosed therein, a polyimide-siloxane block copolymer is applied to selected surface areas of the semiconductor device and cured to provide a tenaciously adherent passivation layer exhibiting good abrasion resistance and excellent dielectric properties. A layer of borosilicate or phosphosilicate glass is then applied by chemical vapor deposition over the copolymer layer as a primary hermetically sealing outer coating. To provide enhanced resistance to chemical attack and to further reduce moisture permeability, a secondary outer glass layer of silox or silicon dioxide is deposited as a top coat over the primary glass layer.
While this multiple layer encapsulation approach of my above-cited U.S. Pat. No. 4,198,444 is sufficiently protective for most power semiconductor device applications, I have found that the afforded degree of impermeability is insufficient for extended operating life of integrated circuitry exposed to certain particularly hostile environments, such as, for example, in the case of human implanted prostheses.